This invention relates to improved mode-locked optical parametric oscillator (OPO) apparatus.
Mode-locked OPO apparatus is useful in generating extremely short pulses (1-0.001 nanoseconds) of light tunable over the oscillator band for various sub-nanosecond spectroscopy phenomena investigations such as the study of rapidly decaying transitions in certain chemical processes. In the past, such tunable mode locking in the visible spectrum has been achieved with dye lasers, but such lasers have outputs with wavelengths limited to about 1 .mu.m or less, i.e., in the visible spectrum. A mode-locked dye laser of this type is described by Ippen, Shank and Dienes in APPLIED PHYSICS LETTERS, Oct. 15, 1972, pp. 348-350.
In addition to the above, sub-nanosecond pulses of light in the infrared spectrum have been produced in the past with solid state laser apparatus in which the effective lengths of the pump and the OPO cavities are identical. In a parametric oscillator, the relationship between the pump, signal and idler frequencies is EQU f.sub.p =f.sub.s +f.sub.i ( 1)
When any of the components are resonated (two are resonated in a singly-resonant OPO and all three are resonated in a doubly-resonant OPO), their frequencies can occur only as a "comb" of equally spaced modes having a separation of c/2l, where c is the velocity of light in vacuum and l is the effective resonator length for that particular component. The reason for the requirement of identical effective cavity lengths for previous OPOs is that this results in combs of modes with equal frequency separations thereby allowing Equation (1) to be satisfied. However, in a doubly-resonant OPO if for any reason either the pump cavity or signal/idler cavity lengths change by a fraction of an optical wavelength, that comb of frequencies will be shifted sideways and the three combs of resonant frequencies will no longer satisfy Equation (1).
As the OPO is tuned away from degeneracy, defined as the condition in Equation (1) at which f.sub.s =f.sub.i, the signal and idler wavelengths change with the former decreasing and the latter increasing. In the above prior art OPO, such tuning rapidly extinguishes OPO operation from two different effects. First, in a doubly-resonant OPO, the signal and idler indices of refraction assume new values after tuning and the signal and idler combs of modes are shifted sideways relative to each other. As a result, the individual cavity resonances no longer have values necessary to satisfy Equation (1).
The second effect occurs more gradually and effects both singly- and doubly-resonant OPOs. As the OPO is tuned well away from degeneracy, the signal and idler wavelengths become well separated. Their indices of refraction therefore differ and the c/2l separation of the signal and idler modes become different. Therefore, it is no longer possible for the combs of pump, signal, and idler modes to interact so as the sustain mode-locked operation of the OPO.
A singly-resonant OPO of the type mentioned above is described in an article entitled "Tunable Infrared Ultrashort Pulses From A Mode-Locked Parametric Oscillator", by Weisman et al, OPTICS COMMUNICATIONS, October 1976, pages 28-32, inclusive.
Another example of a proposed prior art optical parametric oscillator is the doubly-resonant OPO described in the paper of Becker et al entitled, "Analytic Expressions For Ultrashort Generation In Mode-Locked Optical Parametric Oscillators", JOURNAL OF APPLIED PHYSICS, September 1974, pages 3996-4005, inclusive.